This invention relates to core constructions of nuclear reactors and more particularly to a core construction of a boiling-water nuclear reactor having an array of fuel assemblies and control rods and a method of operating the nuclear reactor.
The core of a boiling-water nuclear reactor comprises an array of fuel assemblies and control rods, and the power developed by the reactor is controlled by inserting or withdrawing the control rods. It has hitherto been customary to use those fuel assemblies which are uniform in infinite multiplication factor throughout the length thereof.
In a boiling-water nuclear reactor, an axial void distribution in which the volumetric rate increases in going from the lower region of the core towards the upper region thereof tends to occur during the operation of the reactor to develop power. Thus the use of the fuel assemblies of the aforesaid type of the prior art has caused a skewing in power distribution in which a peak power is developed in the lower region of the core. As a result, withdrawing a control rod has given rise to a sudden change in thermal load applied to the fuel assemblies located adjacent the withdrawn control rod, particularly in those sections of the fuel assemblies which show a peak power. Such change in thermal load has often led to the occurrence of a pellet-clad-mechanical-interaction (hereinafter referred to as a PCMI) between the fuel pellets and the casing or cladding of each fuel rod of the fuel assemblies, thereby causing failure of the fuel rods.
In order to avoid the aforesaid problem, proposals have been made to use fuel assemblies in which a mean infinite multiplication factor is smaller in a lower section of each fuel assembly than in an upper section thereof. For example, Japanese Pat. Appln. Kokai (Laid-open) No. 30790/74 describes fuel elements in which the fuel pellets packed in each cladding have a smaller diameter in the lower section of each fuel element than in the upper section thereof. Japanese Pat. Appln. Kokai (Laid-open) No. 78092/74 discloses fuel elements in which the fuel pellets peaked in each cladding are hollow in the lower section of each fuel element. Also, Japanese Pat. Appln. Kokai (Laid-open) No. 81796/74 discloses fuel elements in which the fuel pellets packed in each cladding have a lower density in the lower section of each fuel element than in the upper section thereof.
Proposals have been made to use fuel rods which contain gadolinea (Gd.sub.2 O.sub.3) or other burnable poison fitted in a portion of each fuel rod in which a peak power is developed.
The use of the aforesaid fuel elements or rods have been instrumental in suppressing, to a certain degree, the development of a peak power in the lower region of the core. However, a change in thermal load applied to the fuel elements caused by the withdrawing of a control rod still exceeds the power of a reactor at which a PCMI is caused, so that the danger of bringing about a PCMI still remains to be eliminated.
The inability of core constructions of the prior art to eliminate the possibility of the occurrence of a PCMI has made controlling the power of a nuclear reactor a complex and time-consuming process. More specifically, the danger of the occurrence of a PCMI has precluded the withdrawing of the control rods freely as desired, with the result that the common practice has been to withdraw the control rods to a certain degree and then lower the power of the reactor temporarily.